Update small typo (#322)

not "substanative" and the word should be corrected to "substantive"

Author: DikaCream

.github/pull_request_template.md

@@ -2,7 +2,7 @@ Before opening your pull request, please respond to the following prompts.
 
 #### Is this resolving a feature or a bug?
 
-_NB: We DO NOT accept typo fixes. Generally, we do not accept edits to comments (starting with `//`) or minor grammatical and technical edits more generally, but do accept substanative fixes and improvements to the content of documentation comments (`///`) and README files._
+_NB: We DO NOT accept typo fixes. Generally, we do not accept edits to comments (starting with `//`) or minor grammatical and technical edits more generally, but do accept substantive fixes and improvements to the content of documentation comments (`///`) and README files._
 
 #### Are there existing issue(s) that this PR would close?